U.S. President Barack Obama spoke with German Chancellor Angela Merkel Friday about the crisis in Ukraine.

The White House said in a statement the president agreed with European leaders on the need for Russia to pull back its forces, allow for the deployment of international observers and human rights monitors to Crimea, and support free and fair presidential elections in May.

The White House statement said Obama and Merkel discussed the need for Russia to "agree quickly" on the formation of a contact group, leading to direct dialogue between Ukraine and Russia "to de-escalate the situation and restore Ukraine's territorial integrity."

Meanwhile, in a telephone conversation Friday with U.S. Secretary of State John Kerry, Russian Foreign Minister Sergei Lavrov warned the U.S. against taking any "hasty and reckless steps" that could harm Russian-American relations. Lavrov said the sanctions would hit the U.S. "like a boomerang."

Earlier Friday, armed men smashed a Russian military truck through the gates of a Ukrainian missile defense post in the Crimean peninsula.

The men negotiated with the base's commanders as soldiers sheltered in their barracks Friday. No shots have been reported.

Initial reports said the truck had crashed through to the base in the port city of Sevastopol and that it was being stormed, but the reporter could not see any signs of major damage to the gates and the base was quiet.

A Ukrainian military official, Vladislav Seleznyov, was quoted by Reuters as saying by telephone that the armed men took over the base without any shooting and that no one was hurt.

Another Ukrainian military official told Reuters at the post that the armed group inside had not seized any weapons and said he was mediating between the sides.

Crimea's pro-Russia premier, Sergei Aksyonov, was asked about the incident during a political chat show being shown live on Ukrainian television. He said all was calm at the military post.

Ukraine's border guards said on Friday that Moscow had poured troops into the Crimean peninsula, where Russian forces have seized control.

Serhiy Astakhov, an aide to the border guards' commander, said there were now 30,000 Russian soldiers in Crimea, compared to 11,000 permanently based with the Russian Black Sea fleet in the port of Sevastopol before the crisis.

The Pentagon estimates there are now about 20,000 Russian troops in Ukraine.

A senior State Department official said Kerry spoke with Lavrov Friday as a follow-up to Obama’s call to Russian President Vladimir Putin on Thursday. He said Kerry underscored the importance of finding a constructive way to resolve the situation diplomatically, which would address the interests of the people of Ukraine, Russia, and the international community. Kerry and Lavrov agreed to continue to consult in the days ahead on the way forward.

Kerry then reportedly spoke with Ukrainian Prime Minister Arseniy Yatsenyuk to debrief him on his conversation with Lavrov, and to stay in closest possible coordination on how the U.S. and the international community can continue to support the people and government of Ukraine.

Meanwhile Ukraine's interim president signed a decree Friday canceling a planned referendum on Crimea joining Russia, but Crimean officials vow the vote will go ahead.

Interim President Oleksandr Turchynov signed the decree Friday, a day after Crimea's Moscow-backed legislature voted for the peninsula to become part of Russia and scheduled a referendum on the issue for March 16.

Ukraine's Prime Minister Arseniy Yatsenyuk speaks during a media conference at an EU summit in Brussels on March 6, 2014.

Turchynov on Thursday called the planned referendum a "farce" and accused the Russian military of organizing the vote. He said he and the Ukrainian parliament would protect the country's integrity and sovereignty. He also said that Ukraine's parliament would initiate proceedings to dissolve the Crimean parliament.

Yatsenyuk said Friday that "no one in the civilized world" will recognize the referendum's results.

Yatsenyuk told reporters he wants to "warn separatists" and others he described as "traitors of the Ukrainian state" that their decisions are "unlawful" and "unconstitutional." U.S. and European leaders have also called the referendum illegal.

But Crimean officials fired back Friday, saying the vote will go forward. The dispute over the vote comes amid reports from Reuters that armed men took control of a Ukrainian missile defense post in Sevastopol on Friday, but no shots were fired.

"Kyiv will not be able to derail the referendum in the Crimea," said Mikhail Malyshev, chairman of the election commission overseeing the referendum on the peninsula. "It will be held, as scheduled, on March 16."

Russia reacts

Russia said on Friday any U.S. sanctions imposed against Moscow over the crisis in Ukraine would boomerang back on the United States, raising the financial stakes as the military standoff intensified.

In a telephone conversation with Kerry, Lavrov warned against “hasty and reckless steps” that could harm Russian-American relations, the foreign ministry said.

Putin, who later opened the Paralympic Games in Sochi which have been boycotted by a string of Western dignitaries, said Ukraine's new, pro-Western authorities had acted illegitimately over the eastern, southeastern and Crimea regions.

“Russia cannot ignore calls for help and it acts accordingly, in full compliance with international law,” he said.

In Moscow, the Speaker of Russia's Upper House of Parliament, Valentina Matviyenko, said Friday that Russian lawmakers will support Crimea's decision if the Ukrainian region decides in a referendum to join Russia, as tens of thousands of people turned out for a rally in the Russian capital to show solidarity with Crimea's pro-Russian authorities.

Meanwhile, a United Nations spokesperson described the recent developments in Ukraine as "worrying and serious" adding that Secretary-General Ban Ki-moon urges the authorities in Ukraine and Crimea to treat the matter with calm, and consider the implications of hasty actions.

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Images from Ukraine

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Obama spoke by phone Thursday with Russian President Vladimir Putin about the Ukraine crisis.

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The White House says Obama told Putin the presence of Russian forces in Crimea is a violation of Ukraine's sovereignty. The Kremlin says Putin denounced Ukraine's new government as "illegitimate" and said Russia cannot "ignore" calls for help from Ukraine's Russia-leaning east and south.

The White House says Obama also called for direct talks between Kyiv and Moscow that would be mediated by the international community. Obama called for all Russian forces to return to their bases and for international monitors to ensure the safety of Ukrainians, including ethnic Russians.

Earlier Thursday, Obama authorized sanctions, including visa restrictions, against those found to have violated Ukraine's territorial integrity. The EU also took measures against Russia, suspending talks on visas and a new economic agreement.

Russia's Foreign Ministry on Friday called the EU's position "extremely unconstructive," adding that Russia "will not accept the language of sanctions and threats" and promising retaliation if the EU imposes sanctions.

Russian news agencies quoted Putin's spokesman Dmitry Peskov as saying in a televised interview that despite "profound disagreements" between the two sides, “the hope remains that as a result of dialogue it will be possible to find some common ground,” and that Russia and the West do not return to a period of conflict like the Cold War.

Still, Peskov dismissed the idea that Western countries could mediate talks between Russia and Ukraine. He also said that those who were, in his words, “behind the coup in Kyiv,” could carry out “purges” in Crimea were they to take control there.

Russia, he added, “cannot remain indifferent, and will not remain indifferent” if a “deadly danger hangs over Russians” anywhere, “especially in neighboring Ukraine.”

Putin denies that the forces with no national insignia that are surrounding Ukrainian troops in their bases are under Moscow's command, although their vehicles have Russian military plates. The West has ridiculed this claim.

Turkey scrambles jets

The Turkish Air Force scrambled six F-16 fighter jets after a Russian surveillance plane flew parallel along its Black Sea coast, the military said on Friday.

The Thursday incident, reported by Reuters, is the second of its kind this week. The Russian plane remained in international airspace, according to a statement on the website of the military General Staff.

NATO member Turkey forms the southern coastline of the Black Sea.

On Friday, a U.S. navy guided-missile destroyer, the USS Tuxton, passed through Turkey's Bosphorus straits bisecting Istanbul on its way to the Black Sea in what the U.S. military described as a "routine" deployment scheduled well before the Ukraine crisis.

Pro-Western Ukrainian opposition icon Yulia Tymoshenko urged Europe Thursday to take strong action to prevent Ukraine's Crimean peninsula from joining Russia, saying such a move would destabilize the entire continent. The EU took measures against Russia Thursday, suspending talks on visas and a new economic agreement.

Tymoshenko said there was a danger of guerrilla war in Crimea should it be incorporated into Russia and appealed to Germany and others on Friday for immediate economic sanctions against Moscow.

Speaking to Reuters after a meeting with German chancellor Angela Merkel, Tymoshenko said international measures against Russia had so far been ineffective and called for immediate action to prevent a "flashpoint."

"As of today, those instruments that have already been applied by the US and the EU didn't produce any tangible effects," she said, summarizing her message to Merkel. "If these instruments do not produce results, there are two options left. To opt for next strongest sanctions, I proposed a set of nonviolent, economic measures." The alternative, she said, was to give Crimea to Russia.

Russia's Foreign Ministry on Friday called the EU's position "extremely unconstructive," adding that Russia "will not accept the language of sanctions and threats" and promising retaliation if the EU imposes sanctions.

Ukrainian Prime Minister Yatsenyuk said Friday his government is "prepared to rebuild relations with Russia." But he said Russia must withdraw its troops, fulfill its agreements with Ukraine and stop supporting separatists in Crimea.

On Capitol Hill, the U.S. House of Representatives voted to provide loan guarantees of $1 billion to Ukraine. That measure now goes to the U.S. Senate. The European Union is prepared to extend a $15 billion bailout to Kyiv if Ukraine can reach an agreement with the International Monetary Fund.

Japan, China

Japan endorsed the Western position that the actions of Russia, whose forces have seized control of the Crimean peninsula, constitute "a threat to international peace and security." The statement comes shortly after President Barack Obama spoke to Prime Minister Shinzo Abe.

China, often a Russian ally in blocking Western moves in the U.N. Security Council, was more cautious, saying that economic sanctions were not the best way to solve the crisis and avoiding comment on the legality of a Crimean referendum on secession.

Cadets withdrawn

Ukraine summoned home its small contingent of cadets and officers studying at military academies in Russia, the Ukrainian Defense Ministry said on Friday, following Russian military action in Crimea.

A statement on the ministry's website said it had canceled a bilateral agreement on military education between the two states. It said 26 Ukrainians were studying in Russian military academies and would be summoned home.

Ukraine's crisis began when protests erupted in late November after then-President Viktor Yanukovych rejected an economic deal with the EU in favor of closer ties with Russia. What began as peaceful protests quickly turned violent, leading to the deaths of more than 80 protesters and charges that the Yanukovych government ordered snipers to shoot protesters. Yanukovych fled Ukraine last month.

Some information for this report provided by Margaret Besheer at the United Nations and Reuters.